A technology called a digital coherent optical transmission utilizes the intensity and phase (and further polarization) of light by digital signal processing to increase the transmission efficiency and hence implement a large capacity long distance transmission. As illustrated in FIGS. 1A to 1C, as a kind of digital modulation, IQ modulation is performed which represents digital data as a symbol point on an orthogonal coordinate having an I axis (an in-phase axis with a carrier wave) and a Q axis (a quadrature phase axis orthogonal to the carrier wave) (see, e.g., FIG. 1C). An IQ modulator modulates light output from a laser diode (LD) into an in-phase optical signal and an orthogonal quadrature phase optical signal by an analog electric signal (see, e.g., FIG. 1A). The analog electric signal represents electric field information of the symbol point. A receiving side extracts a beat signal by causing signal light and locally-oscillated light to interfere with each other to detect information on the phase and intensity of the received light. Since the beat signal is subjected to a digital signal processing such as a dispersion compensation and the waveform of the transmitted signal is identified and reproduced, the reception sensitivity is improved.
In recent years, a technology called a Kramers-Kronig (hereinafter, abbreviated as “KK”) detection method has attracted attention (see, e.g., Non-Patent Document 1 and Non-Patent Document 2). In the KK detection method, signal light is transmitted with a signal waveform including a modulated light spectrum of a certain bandwidth and a continuous light (carrier) spectrum which protrudes at one edge of the modulated light spectrum. A receiving side can restore the phase of a received signal from the output intensity of a photodiode (PD) as long as the received signal satisfies the minimum phase system. The KK detection method enables a dispersion compensation while performing a direct detection, thereby further extending a transmission distance compared to the general direct detection method. It is expected that the KK detection method will be applied to short-distance optical networks and metro networks.
A transmitter configuration using an IQ modulator in the optical transmission of the KK detection method has been reported (see Non-Patent Document 2 as described below). As illustrated in FIG. 1B, for the IQ modulator, a Mach-Zehnder (MZ) interferometer type optical modulator is generally used for each of the I axis and the Q axis. In the MZ interferometer type optical modulator, electrodes for applying a phase change are arranged in the vicinity of a pair of optical waveguides forming an interferometer. In the IQ modulator, two MZ type modulators provided with electrodes are arranged in parallel to give a phase difference of 90° between the I axis and the Q axis. This may cause a problem of complicated configuration such as an increased size of the modulator.
Related techniques are disclosed in, for example, A. Mecozzi, et al., “Kramers-Kronig coherent receiver, “Optica 3, 1220-1227 (2016) (Non-Patent Document 1), and Chen, et al., “218-Gb/s Single-Wavelength, Single-Polarization, Single-Photodiode Transmission Over 125-km of Standard Singlemode Fiber Using Kramers-Kronig Detection,” OFC 2017, Th5B. 6 (2017) (Non-Patent Document 2).